Moisture amounts of the white and red onions of the control group were determined to be 91.59% and 87.32%, respectively. The highest decrease in moisture content occurred within the red onion (7.14%) dried at 90°C. The total phenolic, total flavonoid contents, and antioxidant activity values of the white and red onions dried at both temperatures increased significantly compared to the control groups. The highest increase for total phenolic and total flavonoid contents were established for white and red onions dried at 90°C. Caffeic acid, rutin-trihydrate, p-coumaric acid, trans-ferulic acid, apigenin-7-glucoside, quercetin, and trans-cinnamic acid contents of both the undried and dried red onions were found to be higher compared to the contents of the undried and dried white onions. Additionally, the amounts of phenolic constituents of white and red onions dried at 50 and 90°C temperatures were higher compared to those of the control groups. Practical applications: Various types of dried onions forms include flaked onion, minced onion, chopped onion, and powdered onion, and are a product of substantial prominence and importance across the global trade landscape. Moreover, the onion plant enjoys widespread use as a flavor additive in copious amounts of food formulations including comminuted meats, sauces, soups, and salad dressings. By drying the plant materials, the moisture content of the product is brought to a suitable value for storage, and in addition to this, it is to prevent the losses in the flavor, taste, color, and nutrients of the vegetable materials.